The invention relates to liquid heat transport systems for use in space reactor power systems and more particularly to means for gradually increasing the rate of fluid flow resulting from thawing the liquid-metal in a heat transport system for use in space reactor power systems.
Liquid metals are used as circulants in space reactor power systems both for primary heat transport and for waste heat removal. Many desirable coolants are in a frozen state at original startup and require a means to thaw the coolants before flow circulation can be established. Lithium is a prime example of a coolant material of this type. In current space reactor power system designs, liquid lithium is used to transport heat from the reactor at approximately 1100.degree. C. and to remove waste heat from energy conversion units at 500.degree. C. Lithium, however, has a melting temperature of 180.degree. C. and requires a means of thaw before full flow circulation can be attained.
Certain liquid metals, such as the eutectic combination of sodium and potassium (NaK), are molten at low temperatures and would obviate the need for thaw. However, available eutectic combinations are heavy and require considerable pumping power for effective circulation. A further consideration relates to the safety hazards of launching into space systems with the circulants in a molten state. The possibility of launch-induced leaks would impose a serious hazard to both crew (in the case of a shuttle launch) and equipment. The use of lithium circulant precludes this possibility because it would remain frozen during the entire launch phase. For those performance and safety reasons, lithium is the preferred circulant in many high-performance systems.
U.S. Pat. No. 3,537,515 discloses a circulatory cooling system utilizing liquid-metal coolant lines which are heated by a series of heat pipes butted end-to-end and extending through the center of the lines. The heat pipes derive their heat directly from the waste heat of the heat source.
In certain implementations the reactor is a source of heat that activates fluid circulation pumps by thermoelectric means. Thus, during the thaw process, the pumps will develop pumping head although no circulation is possible until full thaw is achieved with the removal of blockage by frozen material. With relatively high reactor temperatures required to deliver heat during the thaw process, full thaw may be accompanied by a sudden flow as the final blockage is removed, giving rise to two significant problems: (1) the hot flow to the power conversion system may result in severe thermal gradients with high potential for thermal shock; and (2) the relatively cool return flow from the power conversion system to the reactor may result in large changes in reactivity with consequent reactor control instabilities.
It is possible to minimize these effects by modulating the pumping action either electrically or mechanically. These methods, however, introduce complexity and defeat the benefits of the essentially passive nature of thermoelectrically-driven pumps.